Women with Alzheimer’s may face a swifter mental decline than men with the same condition, but researchers are not sure why, according to a study released this week at US a medical conference.

Some two-thirds of US seniors living with Alzheimer’s disease are women, and women are almost twice as likely as men to develop the incurable cognitive disease, according to the Alzheimer’s Association, which is hosting is annual conference in the US capital.

On Tuesday, researchers from Duke University presented findings on a study of 141 women and 257 men, aged in their mid 70s, who suffer from Alzheimer’s disease.

After studying the group for eight years, they found that women’s cognitive abilities declined twice as fast as those of men, judging by mental tests taken each year to gauge memory and other skills.

“Our findings suggest that men and women at risk for Alzheimer’s may be having two very different experiences,” said lead author Katherine Amy Lin at Duke University Medical Center.

“Our analyses show that women with mild memory impairments deteriorate at much faster rates than men in both cognitive and functional abilities.”

The reasons behind the difference remain unclear, and more study is needed to determine if there are gender-specific genetic or environmental risk factors at play.

“Women are disproportionately affected by Alzheimer’s, and there is an urgent need to understand if differences in brain structure, disease progression, and biological characteristics contribute to higher prevalence and rates of cognitive decline,” said Maria Carrillo, Alzheimer’s Association chief scientific officer.

Alzheimer’s disease affects some 44 million people around the world, and cases are expected to skyrocket as the global population ages.